Many attempts have been made to provide containers for processing and storing foods which permitted heating by microwave and some, also, by conventional ovens. For example, in U.S. Pat. No. 3,865,302, Kane discloses a container consisting of a polyethylene terephthalate tray having a polyethylene terephthalate lid bonded to it with an adhesive blend of an ethylene copolymer and a wax. The tray is formed of a single ply film employing titanium dioxide as an opacifier and talc to enhance the rate of crystallization. The lid, made of a material having properties similar to the tray, is bonded to a flange on the tray by a heat-activated adhesive. The adhesive does not flow or leak even after cooking at temperatures of 400.degree. F. Unfortunately, containers which remain sealed when oven heated to 400.degree. F. develop significant internal steam pressures.
The problem with steam buildup can be especially severe during microwave heating because of the very nature of the heating phenomenon. Microwave heating virtually always causes steam generation from hot spots before the food is heated desirably throughout. The continued generation of steam at these hot spots and the continued heating of the steam itself cause pressures which must be released in a controlled manner. In U.S. Pat. No. 4,210,674, Mitchell proposes applying a small strip of metal to the lid to melt a vent during heating. Similarly, in U.S. Pat. No. 3,997,677 Hirsch and Spiegel disclose a covered tray-like package which includes a weakened seal area to provide pressure relief. Also, several patents have disclosed ovenable polymer-coated paperboard packages which would self vent when heated. For example, in U.S. Pat. No. 3,863,832 Gordon and Siegele disclose that the seal can be made to fail at a preselected temperature where the coating is polyethylene or polypropylene for both tray and lid. Similarly, in U.S. Pat. No. 4,261,504, Cowan discloses a self-venting package having polymer coated paperboard upper and lower portions. One is given a coating of thermoplastic polyester and the other, a thermosetting polyester to provide a package which is said to vent by steam buildup just prior to completion of the cooking cycle.
Another concern, but one which is not addressed by the above patents, is the development of a package which can be used for retorted foods which are able to be stored at room temperature. For example, in U.S. Pat. No. 4,367,312, Bontinck, Jacobs and D'Hondt discuss as important, the ability to form a peelable seal which withstands retort conditions. To achieve a retortable, but peelable seal with packaging films including polyester, polyvinylchloride, polyamide, and polyacrylonitrile, they disclose a blend of an ethylene polymer with polystyrene and other optional components. Additional polymers are disclosed to enhance interpolymer compatibility, fillers can be added for opacity or hardness, and adjuvants and the like can be added for their functions. The heat sealable films are prepared by extrusion followed by corona discharge pretreatment.
The retorting process for non-rigid containers typically entails heating them in water or steam to a predetermined temperature, holding them at that temperature for a time required to thoroughly heat the contents, and then cooling. The pressure inside the containers varies in response to external temperature changes. Unless the proper materials of construction are employed and unless this pressure is compensated for by causing the external pressure to approximate that on the interior, a variety of faults and failures will be produced, among which are: bursting of the packaging material; wrinkles in flexible film portions, especially at corners; partial release of the seal along seams; distortion of printed matter; and other defects.
One commercial packaging system employs three distinct members which cooperate to deliver a room temperature storable, retorted, prepared food product which can be prepared for serving by microwave or conventional oven heating. This package has a shallow dish-like bottom portion, a multi-ply lid heat-sealed to a top flange on the dish, and a vented dome cover which snaps over the top of the dish. To achieve good strength and heat sealability to the lid, the tray is formed of two layers of unpigmented polyethylene terephthalate. The outer layer is partially crystallized to improve toughness and barrier properties while the inner layer is highly amorphous for better heat sealing. The multi-ply lid is a sandwich of polyethylene terephthalate films bonded to an inner aluminum foil core. One side of the lid is coated with an adhesive to provide a peelable seal with the tray flange. In use, the tray is filled with a prepared food, the film is heat-sealed to the tray, and the sealed tray is then heated in a retort as required while applying an external pressure to counterbalance the internal pressure. To prepare the product for use by microwave oven, the heat-sealed lid is removed and discarded and the vented dome cover is snapped onto the tray. In contrast, when the food is prepared for serving by conventional oven heating, the vented dome is discarded and the heat-sealed lid peeled back for a short distance.
The use of both the heat-sealed lid and a separate vented cover is an inefficient use of packaging material, inconvenient, and the possible cause of accidents arising during preparation. Accordingly, while the art has long sought a package which would fulfill the needs for simplicity and effectiveness for foods which were to be capable of final preparation by microwave ovens, it has yet to achieve this. The need is especially acute and unfulfilled for foods which must be retorted to provide room-temperature, shelf stability.